IO LIBBIE II. HYMAX. 



as already mentioned. After disintegration has begun at the 

 posterior end it proceeds forwards, in the neural tube first, 

 then in the somites; the last region of the embryo to disintegrate 

 is in general at about the level of the middle somites. The 

 anterior somites are commonly more susceptible than the pos- 

 terior ones and susceptibility in the somites decreases from both 

 ends towards the middle. The limb buds in the three-day chick 

 are regions of moderately high susceptibility, the anterior limb 

 buds being more susceptible than the posterior ones. The ear 

 vesicles are more susceptible than the adjacent region of the 

 brain in the two-day chick but their susceptibility is decreased 

 in the three-day chick. The visceral arches in all stages in 

 which they could be observed with certainty were the least 

 susceptible parts of the embryo; they appeared to disintegrate 

 from the posterior end forwards as do the somites in early stages. 



In these later stages of the chick evidences of a medio-lateral 

 gradient were noted. If we consider a section through the middle 

 of the body, the neural tube is the most susceptible part of the 

 section; the medial parts of the somites disintegrate next; and 

 the disintegration then proceeds laterally along the somites to 

 and along the lateral plate. In two- and three-day chicks where 

 the first somites present their lateral surfaces to the observer, 

 owing to the turning of the trunk, it could easily be seen that the 

 medial portions of the somites are more susceptible than their 

 lateral portions. 



75. Entodermal Structures. The posterior part of the noto- 

 chord can usually be seen more or less distinctly. This part dis- 

 integrates from the primitive streak anteriorly. The anterior 

 part of the notochord is more or less obscured but appears to 

 disintegrate from the diencephalon caudally. The gradient in 

 the notochord is thus probably of the double type as in the later 

 somites and the neural tube. 



If the embryo is mounted ventral side up, the ventral layer 

 of the blastoderm, which is of course composed of entoderm, 

 is exposed to view but is of so delicate a texture that its time of 

 disintegration is difficult to determine. In the posterior part of 

 embryos up to two days of age the entoderm layer also appears to 

 disintegrate from the posterior end forward, thus accompanying 



